This invention relates to a microwave applicator for the treatment of a body by means of microwave electromagnetic energy. The body is preferably biological tissue and, preferably, the applicator is for use in the treatment of menorrhagia.
Menorrhagia is a common condition in women over the age of forty and manifests itself as excessive bleeding from the endometrium which constitutes the inner wall of the uterus.
The most common form of treatment is to carry out a hysterectomy in which the entire uterus is removed.
In our earlier application published under number WO95/04385, the contents of which are incorporated herein by reference, we disclosed a probe for applying electromagnetic radiation at microwave frequency which comprised a dielectric-filled waveguide with an exposed portion at the tip defining an antenna. However, in several of the embodiments, the microwaves were launched in a first air-filled waveguide and then the microwaves were passed into a second waveguide which contained the dielectric material. Between the waveguides, a tapered waveguide provided a transition. The dielectric filled waveguide was of smaller diameter than the air-filled waveguide because, at a given frequency, the wavelength in dielectric is shorter. Hence the diameter of the applicator in wavelengths remains constant throughout transition.
However, although such a applicator is perfectly satisfactory, the applicator bandwidth is comprised by the resonance found in the long length of dielectric filled waveguide. This means that any change in frequency generated by the microwave source could make a significant difference in applicator efficiency.
According to the present invention, there is provided a microwave applicator for applying electromagnetic radiation at microwave frequency comprising a coaxial input for receiving and passing a microwave signal input of predetermined frequency, a waveguide for receiving and propagating the microwave signal input, dielectric material positioned within the waveguide and extending beyond the waveguide to form an antenna for radiating microwave energy, characterised in that the coaxial input has means for providing a direct, in-line transition of the microwave signal input into the dielectric-filled waveguide.
Preferably, this direct in-line transition is achieved by the central conductor of the coaxial input extending axially centrally into the waveguide so as to excite microwaves in the waveguide. A lateral conductor extends radially from the central conductor towards the outer wall of the waveguide and serves to assist the launch of the microwaves into the waveguide in the appropriate mode for transmission to the tip.
Preferably, the applicator includes a temperature sensor which is directly connected to the coaxial input to minimise wiring.
Suitable, where the applicator is to be used for medical treatment such as endometrial ablation, it is important that the applicator be sterile for each use. Accordingly, preferably the applicator is coated with a microwave transparent coating allowing the applicator to be cleaned in conventional manner.
Although the microwave applicator of the present invention may be used for any desired application, it is preferred that it be used for endometrial ablation. This requires applying microwave energy to the applicator at a frequency which will be substantially completely absorbed by the endometrium, monitoring the operating temperature to ensure that the endometrium tissue is coagulated evenly through the uterine cavity, thus maintaining the application of the microwave energy for a period of time sufficient to destroy the cells of the endometrium.
The use of microwave power to heat the endometrium has two main advantages. Firstly, electromagnetic radiation at microwave frequencies is strongly absorbed by tissue and at around 8-12 GHz all microwave power is absorbed in a layer of tissue about 5 mm thick and it is impossible for microwave heating to extend beyond this region. This is ideal for the treatment of the endometrium which is about 5 mm thick. Secondly, because of this strong absorption, the amount of power required to achieve the desired temperature is relatively small.
Moreover, the improved applicator of the present invention has the following major advantages over the applicator previously disclosed in our aforementioned earlier application:
(i) the waveguide is shorter because, by forming a hybrid between a coaxial input and a dielectric filled waveguide, the distance between the transition and the radiating tip is very much shorter. This, in turn, reduces the amount of dielectric material necessary which improved band width and applicator efficiency; and
(ii) it is possible to make the applicator flexible.